局部宽浅型蚀坑的简化表征形式研究

STUDY ON SIMPLIFIED CHARACTERIZATION FORM OF LOCAL WIDE AND SHALLOW CORROSION PITS

为评估24个钢材试件上局部宽浅型蚀坑的合理简化形式,分别选取半椭球体、球冠形两种常用的简化表征方法,先推导了蚀坑的体积、横断面轮廓线曲率半径计算式;再分析电化学加速锈蚀试验误差,对比4个典型坑蚀钢材试件单轴拉伸有限元分析结果与其实际疲劳断裂情况。结果表明,在一定的通电时间下,与由法拉第定律计算的预定锈损体积相比,由实测蚀坑尺寸得到的半椭球体蚀坑体积最大偏差达到53.2%,平均偏差为20.26%;而球冠形蚀坑体积最大偏差仅32.3%,平均偏差为12.8%;半椭球体蚀坑最大应力点位于蚀坑口边缘;而球冠形蚀坑最大应力点大部分位于坑底,仅深径比较大时位于坑底与坑口连线上偏向坑口的三分点附近,所指示的危险区域与试件疲劳断裂过程中主裂纹萌生位置吻合得更好。两种简化形式下,试件上蚀坑体积、横断面轮廓线曲率半径差异随深径比减小而增大,其中后者相对偏差可达90%以上,是造成应力分布差别较大的主要原因。

In order to evaluate the reasonable simplified form of local wide and shallow corrosion pits on 24 steel specimens,two commonly used simplified characterization methods of semi⁃ellipsoid and spherical cap shape were selected respectively.Firstly,the calculation formulas for the volume of corrosion pits and the curvature radius of the profile of the cross section were derived.Then,the test errors of electrochemical accelerated corrosion were analyzed,and the uniaxial tension finite element analysis results of four typical steel specimens with corrosion pits were compared with their actual fatigue fracture situations.The results show that under a certain power on time,compared with the predetermined rust volume calculated by Faraday′s law,the maximum deviation of the semi⁃ellipsoidal corrosion pits′volume obtained from the measured sizes of corrosion pits reaches 53.2%,with an average deviation of 20.26%,while the maximum deviation of the spherical cap shaped corrosion pits′volume is only 32.3%,with an average deviation of 12.8%.The maximum stress points of the semi⁃ellipsoidal corrosion pits are located at the edge of the top,while the maximum stress points of the spherical cap shaped corrosion pits are mostly located at the bottom.Only when the ratio of depth to radius is larger,it is located on the trisection point of the connecting line between the bottom and the top,near the pit mouth.The danger areas indicated by the spherical cap shaped corrosion pits are in better agreement with the initiation positions of the main cracks in the fatigue fracture process of the specimens.Under two simplified forms,the differences between the pits corrosion volume and the curvature radius of the cross section profile on the specimens increase with the decrease of the ratio of depth to radius,and the relative deviation of the latter can be more than 90%,which is the main reason for the larger differences in the stress distribution.